Reading stations of facsimile transmitters generally comprise photoelectric sensors which respond to the differences in brightness between the background of the carrier surface and the graphic characters appearing thereon, the latter being usually of reduced luminosity. It is customary to measure relative brightness, or image contrast, rather than absolute brightness in order to take the luminosity of the background into account. Such a sensor produces a signal voltage of analog type which can be converted, by comparison with a threshold, into a binary output signal whose two levels characterize the various elemental areas of the explored carrier surface as being of either high ("white") or low ("black") luminous intensity.
If the contrast level is high enough, e.g. on the order of 0.4 to 0.6, conventional systems operating with photoelectrictransducers perform quite satisfactorily. If this requirement is not fulfilled, scanning by a video camera may be used to allow electronic intensification of contrasts; such systems, however, are complex and costly, apart from necessitating frequent readjustments and recalibrations. Similar drawbacks are encountered in systems in which the documents are scanned by a laser beam. There is also the disadvantage that the document must be stationary during scanning, thus permitting only an intermittent advance with resulting strain on the transport mechanism and noisy operation.